Hydrogen evolution reaction metal interface process

If these conditions are not satisfied, some process will be involved to prevent accumulation of the intermediates at the interface. Two possibilities are at hand, viz. transport by diffusion into the solution or adsorption at the electrode surface. In the literature, one can find general theories for such mechanisms and theories focussed to a specific electrode reaction, e.g. the hydrogen evolution reaction [125], the reduction of oxygen [126] and the anodic dissolution of metals like iron and nickel [94]. In this work, we will confine ourselves to outline the principles of the subject, treating only the example of two consecutive charge transfer processes O + n e = Z and Z 4- n2e — R. 

If the corrosion product on the metal surface is also an electronic conductor, the corrosion product does not hinder the flow of electrons, and the electrochemical reaction, instead of taking place at the metal/solution interface, occurs at the corrosion product/solution interface. In this case, if the process is under cathodic control, its total current may be also greatly increased by the presence of the corrosion product both for the possible greater catalytic activity of the corrosion product compared to that of the metal on the cathodic process (e.g., in the case of some iron sulfides with respect to hydrogen evolution) and for the much higher surface area of the corrosion product compared to that of the metal. On the other hand, if the corrosion product does not have the characteristics of an electronic conductor, the electron cannot flow across the corrosion product/solution interface, and the cathodic process occurs only on the limited free metal surface through the porosity of the corrosion product and with hindered diffusion. In the latter situation, the current density of the cathodic process has an upper limit, and it is drastically reduced. 

Most corrosion processes, e.g., metal dissolution, hydrogen or oxygen evolution, and passive film formation, involve at least one adsorption step as a part of the overall reaction. This step can be significantly affected by the presence on the metal surface of a monolayer of nonmetal species. As evidenced by studies described in this chapter, adsorbed species may act by loosening the metal-metal bond or changing the electric field at the metal-electrolyte interface. They can also favor or inhibit the adsorption or the recombination of adsorbed atoms normally involved in the anodic or cathodic reactions. 

---